The claim behind "theory of mind" is that certain core conceptions organize and enable our everyday understanding of the social world. In particular, social cognition is based on thinking of people in terms of their mental states-their beliefs, desires, hopes, goals, and inner feelings. This everyday assumption of mind is powerfiil and constraining. It leads us to try to use the mind and increase its powers, to share inner experiences, to distinguish between purely imaginary and real events, and to interact with other persons by searching for and reaching out to their underlying mentalities. A mentalistic construal of persons is fundamental not only to adults;essential parts of it develop early in childhood. This raises intriguing questions: (1) When do children know what about basic mental-psychological states- intentions, beliefs, desires, emotions? (2) What conceptual progressions or sequences characterize development, for normally developing and delayed individuals? (3) How does change occur: How are these conceptions developed and socio-culturally transmitted;what factors shape understanding;to what extent is a mentalistic construal of persons widespread across cultures vs. lirnited to our society? (4) To what extent are theory of mind understandings specially supported in the brain;how are such brain mechanisms assembled and revised developmentally? The proposed research encompasses multiple interrelated investigations designed to address these questions. The investigations include meta-analytic, conversational, laboratory, and neurophysiological studies that build on and advance my recent related research, and that are designed to answer questions about sequence, change, cultural differences and similarities, and brain bases of our everyday theory of mind. The studies focus especially on infancy and the preschool years, a period of crucial change and transition for theory-of-mind understandings, and a period that establishes foundational conceptual achievements that enable and constrain cognitive development in later childhood and adulthood. RELEVANCEi(See Instructions): Humans are a social species;understanding and navigating the social world is crucial to soimd cognitive, socio- emotional, and physical health. The current research provides critical information about successful and impaired social understandings and informs intervention and training efforts promoting enhanced social understanding and interactions for infants, toddlers and preschoolers in circumstances of typical and atypical development.